Calcium chloride is used in different applications, some of which require “food-grade” calcium chloride that contains low concentrations of fluorides and other contaminants. For example, calcium chloride is used in bisphenol-A plants to break the hydrochloric acid/water azeotrope in hydrochloric acid recovery columns. In this particular application, fluoride ions will concentrate and convert to hydrogen fluoride in the HCl recovery column. Hydrogen fluoride, known to dissolve glass, creates pin holes in the recovery column, disrupting the recovery process and creating leakage problems. “Food grade” calcium chloride is also used in actual food applications, which naturally require high quality materials.
The fluoride concentration in “food-grade” calcium chloride is typically less than 10 ppm. However, this grade of calcium chloride is often difficult to obtain and is therefore expensive. It would thus be desirable to remove the fluoride ions from the calcium chloride solution prior to its use in applications requiring low-fluoride, or “food grade” quality calcium chloride. Many present methods for removing fluoride ions from process and wastewater streams are inadequate or cost prohibitive for obtaining the desired fluoride-free calcium chloride solution because they are inapplicable when calcium and chloride concentrations are high. U.S. Pat. No. 6,355,221 to Rappas and U.S. Pat. No. 5,403,495 to Kust et al. teach the use of calcium fluoride as a seed for creating enhanced calcium fluoride particles in order to remove soluble fluoride from the wastewater streams.
The use of adsorbents to remove fluoride ions in solution has also been effective under certain conditions. For example, European Patent No. EP0191893 to Nomura et al. discloses contacting a solution containing fluorine compounds with various hydrated rare earth oxide adsorbents. Similarly, International Publication No. WO 98/10851 teaches the removal of fluoride ions in solution by passing the solution through an ion exchange resin to produce an ultrapure hydrofluoric acid. However, these methods do not solve the problem of removing fluoride ions from solutions containing high calcium and chloride ion concentrations, thereby generating a purified calcium chloride stream for use in later processing. These methods also do not produce a calcium chloride solution with as little as less than 1 ppm of fluoride.
It would also be advantageous to have an easy, cost-effective method of manufacturing low fluoride calcium chloride.